At 03:56 PM 11/29/2012 +0000, you wrote:
> By theory, yes... but we use a sound card with a lot of flicker noise
on the lower
> end, also we have the 10...20Hz low freq. cutoff due the usage of a
servo / single
> 5V power. Also the raising noise below 100Hz (ADC serve & noise on the
ADC power /
> input circuit) limits the performance.
One way to avoid the problem in the 1/f noise region is to avoid it.
Mix down to a few kHz and analyze you signal or if you must mix down
to "DC" then chop it to bring it up to a kHz or so and analyze it using
a program such as spectrum lab or in software to internally mix back down
to "DC" using software digital mixer and then analyze.
You can use a MOSFET analog switch to chop the signal.
When you chop it you will have an carrier with upper and lower sidebands.
You can simply take the FFT data and either shift the bins making the
carrier (chopping frequency)
the Zero or DC bin or even better yet add the upper and lower sidebands so
that you get a 3db improvement
in S/N. I will be db since the sidebands are identical mirror of each
other, ie voltage doubles when they
are added increasing the signal by 6db but noise introduced by electronics
on upper and lower sidebands
are not coherent and only increases by 3db.
73
Bill wa4lav
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